Add a process CreateBackorderInvoice with no input and the following 3 rules in it:
1) FIND InvoiceLineItem WHERE InvoiceLineItem.QtyBackorder > 0
2) CREATE Invoice
3) CreateBackorderInvoiceLineItem
Add process CreateBackorderInvoiceLineItem with objects Invoice and InvoiceLineItem as its input. The object should have the following single rule:
CREATE InvoiceLineItem WITH InvoiceLineItem.Invoice = Invoice, InvoiceLineItem.QtyBackorder = ThisInvoiceLineItem.QtyBackorder, InvoiceLineItem.ClientID = ThisInvoiceLineItem.ClientID
To create a backorder invoice, start process CreateBackorderInvoice. Rule #3 of process CreateBackorderInvoice will be called for each item found in rule #1. Process CreateBackorderInvoiceLineItem will make a copy of the given item and attach the new item to the new invoice.